The production of hydrocarbons from carbon monoxide and hydrogen (the Fischer-Tropsch process) is carried out in the presence of catalysts based on metals selected from the Group VIII of the Mendeleev's Periodic Table of the Elements. The catalyst composition plays a key role in the production of hydrocarbons because it gives a final result, i.e. product composition.
It is well-known that the production of hydrocarbons from COH2 is exothermic and is carried out at high pressures. There is a need in improvement of the catalyst composition to maintain high activity and selectivity of catalysts. Such improvement will give an option cutting the likelihood of local overheating which has an adverse effect on catalyst selectivity for production of the main products and causes the catalyst deterioration.
The main requirements for the catalytic bed formation in the Fischer-Tropsch process (e.g. high concentration of catalytically active component in the reaction volume, small size of the catalyst particles (less than 50 μm); high heat conductivity of the catalyst bed; the extended surface of the gas-liquid interphase; providing regimen of the gas convection current close to plug flow) are not fulfilled in the current process flow diagrams where the usual catalysts in slurry, fixed or fluidized bed are used (Hasin A. A. et al., Catalysis in industry, No 2, 2002, p. 26-37). Therefore the effective production of hydrocarbons from synthesis gas requires development of the new type catalysts.
The above- described problem takes place in the process on a solid catalyst (pelletized, ring-shaped and the like) which forms the fixed bed and is placed inside the tube divided the gas space with the catalyst and the liquid phase (water) for heat removal. One of the methods for overcoming the process problem is increasing of the heat conductivity of the solid catalyst. It is possible to increase the heat conductivity of the solid catalyst by using of metals, carbides and nanocarbon material as catalyst components (S. Berber et al., Unusually High Thermal Conductivity of Carbon Nanotubes, Physical review letters, Vol. 84, N. 20, 2000, p. 4613-4616).
WO2004069407 discloses a preparation method of a catalyst for production hydrocarbons and/or oxygen-containing compounds from synthesis gas. The catalyst is prepared from powders of a catalytically active agent, a heat-conducting agent and a pore-forming agent with particle size less than 300 μm. Firstly the powders of the heat-conducting and pore-forming agents are mixed, then the powder of the catalytically active agent is added to the mixture, the mixture is compressed, the catalyst body is putted into the required shape and the heat treatment of the catalyst body takes place. The compressing and shaping the catalyst body into cylinder or perforated cylinder or plate or profiled plate is carried out by pelletizing in a rolling mill; the blanking of the plate of the required form is added. The heat treatment is two-stage, firstly in the inert gas current at temperature above 400° C. and then in the hydrogen-containing gas current at temperature above 300° C. The catalytically active agent comprises a metal selected from the Group VIII of the Mendeleev's Periodic Table of the Elements in amount of at least 2 wt %. The metallic copper and/or zinc and/or aluminium and/or tin and/or mixtures or alloys thereof are used as the heat-conducting agent. An oxide and/or hydroxide and/or carbonate and/or hydroxocarbonate and/or a salt of the metal from the heat-conducting agent or the powder of the catalytically active agent are used as the pore-forming agent. The weight content of the pore-forming agent to the weight content of the heat-conducting agent ratio is 0.25-4. The disadvantage of the catalyst is a disalignment of the catalyst body position and the direction of the reaction stream. Therefore in spite of the heat-conducting agent presence such catalyst has low efficiency (CO conversion does not exceed 15% at syngas hour space rate 930 l/h).
EP0681868 relates to a catalyst for the Fischer-Tropsch process. There is cobalt or iron loaded on a support in the catalyst. The support is carbon with the specific surface area at least 100 m2/g. The catalyst comprises a promoter—platinum (0.2-10% ). The catalyst is prepared by impregnation of a carbon powder (0.5-1.0 mm) with an aqueous solution of metal salts. Previously carbon from the organic material (such as coconut coal, peat, coal, carbonized polymers) is treated subsequently at a temperature of 300-3300° C. in an inert, oxidizing and again in inert atmosphere. The production is carried out at a temperature 150-300° C., pressure 0.1-5 MPa and H2:CO ratio of 1:1-3:1. The drawback of the catalyst is low selectivity of C5 and higher products due to the quite low heat conductivity of the support.
SU1819158 provides a catalyst for production of hydrocarbons from synthesis gas. The catalyst comprises iron as active component, copper, silicon, potassium and coal (2-20 g on 100 g of iron) activated with steam or mineral acid. The catalyst is obtained by individual dissolving iron and copper in nitric acid at elevated temperature, then they are mixed and the obtained solution is brought to the boil, alkali liquor or calcined soda solution is added to the boiled suspension to adjust pH to 7-8. The suspension is filtered, the solids are suspended in a steam condensate and potassium containing waterglass is added, followed by nitric acid treatment, the catalyst precipitate separation, drying and formation by extrusion, additional drying and desintegration. The Fischer-Tropsch synthesis is carried out in a reactor with a fixed bed of the catalyst under a pressure 20-30 bar and a temperature 220-320° C. The yield of the solid product in the form of wax is 40-55% on hydrocarbons C2+ basis. The drawbacks of the catalyst are low productivity and selectivity for the main products, as well as a quite difficult preparation method.
The catalysts comprising a carbon nanofiber or carbon nanotubes (CNT, SWNT, MWNT) are well-known.
Carbon nanofiber is a material consisting of thin threads, formed mainly by carbon atoms, each thread less than 1 μm in diameter. Carbon atoms are united into microcrystals, aligned in parallel each other. Carbon fibers are characterized by high tensile strength and chemical resistance, low specific density and coefficient of thermal expansion. Some carbon fibers have higher heat-conductivity along the fiber axis. Carbon nanotube (SWNT, MWNT) is one of the forms of the carbon nanofiber and has maximum heat conductivity. The microstructure of carbon nanotubes differs from the structure of other carbon nanofibers by extended cylindrical structures. These structures have a diameter from one to several dozens nm and they are usually up to few μm in length.
An ideal nanotube is a graphite layer rolled up into a hollow cylinder; the graphite layer is composed of hexagons and every vertex of such hexagon is the carbon atom. However the structure of the experimental single-wall carbon nanotubes is not ideal in many ways. Multiwall nanotubes differ from the single-wall nanotubes by a wide range of shapes and configurations. There are different structures both in the longitudinal and transverse directions. Multi-wall carbon nanotube structure formation depends on synthesis conditions in the specific experiment. An analysis of experimental data demonstrated that the most typical structure of the multi-wall carbon nanotube is a structure like a Russian doll in which tubes of smaller diameter are coaxially arranged within tubes of bigger diameter.
Carbon nanotubes were first synthesized by evaporation of graphite in the arc discharge. In accordance with the present invention carbon nanotubes are formed by chemical vapor deposition (CVD). During CVD, a substrate is prepared with a layer of metal catalyst particles (most commonly nickel, cobalt, iron, or a combination thereof). The substrate is heated to approximately 600-1200° C. To initiate the growth of nanotubes, a carbon-containing gas (such as acetylene, ethylene, ethanol or methane etc.) is bled into the reactor. Nanotubes begin to grow at the sites of the metal catalyst.
Therefore it will be attractive to use carbon nanofiber and nanotubes in the catalyst composition; it makes possible to increase the heat conductivity of the catalyst and have a good influence on the catalyst efficiency.
However the catalysts comprising carbon nanofibers or nanotubes have disadvantages. Particularly carbon nanofibers and nanotubes are corrodible because of hydrogenation. Should the arrangement of carbon nanofibers and nanotubes in the catalyst provides contact of the nanomaterial mainly with hydrogen-containing gas rather than main products, the disadvantage may come out. Although carbon nanofibers and nanotubes have high heat conductivity, they are not able to transfer full heat flow to the surrounding particles by reason of low contact heat conductivity (in the event of free arrangement). It leads to the second disadvantage. Moreover active components of the catalyst have low activity, if the major part of the surface (for loading of the active components) is carbon that is not mixed and impregnated with other components, e.g. oxide components.
For example, CN101185904 relates to a catalyst which is applicable for the selective catalytic hydrogenation of carbonyl compounds to aromatic alcohols and preparation method thereof. The catalyst comprises 1) structured substrate, such as metal foam, honeycomb ceramic, carbon felt and ceramic fiber; 2) nanomaterial coating, such as carbon nanofiber; 3) metal active component, such as nickel, ruthenium, rhodium, palladium and/or platinum.
The main drawback of the catalyst is low activity because the active centers of the catalyst are loaded directly into nanocarbon arranged in the catalyst in such way that the nanomaterial contacts mainly with hydrogen-containing gas. Also a loss of the catalyst in the hydrogenation process of the nanocarbon coating is significant.
EP1782885 discloses a carbon nanotubes supported cobalt catalyst for converting synthesis gas into hydrocarbons. The catalyst is prepared by incorporating cobalt and/or ruthenium and optionally an alkali metal onto a CNT support. The catalyst is suited for the conversion of synthesis gas into a mixture of essentially linear and saturated hydrocarbons. The cobalt content, expressed by weight % of cobalt based on the total weight of the catalyst is between 1 and 60% , the ruthenium content, expressed by weight % of ruthenium based on the amount of cobalt present in the catalyst, is between 0.1 to 1% and the alkali metal content, expressed weight % of alkali metal based on the amount cobalt present in the catalyst, is between 0 to 3% by weight.
The main drawback of the catalyst is an absence of the possibility to use the catalyst in industrial conditions. High yield of the product may be achieved only at low syngas load because the heat released during the process may be removed from the active centers and nanotubes together with the products. The purpose of using CNT in the catalyst is limited by obtaining of the dispersed cobalt clusters.
RU2325226 provides a catalyst for synthesis of hydrocarbons from CO and H2 comprising a metal of the VIII group of the Mendeleev's Periodic Table of the Elements and a support, comprising an oxide component and carbon nanofiber. The active component content is 5-40% by weight based on the total weight of the catalyst and the oxide component contains aluminium oxide and/or silicon oxide and/or titanium oxide and/or zirconium oxide. Additionally the catalyst can include promoters (zirconium and a metal of the VII, VIII groups of the Mendeleev's Periodic Table of the Elements and/or oxides in an amount of 0.1-5% by weight based on the total weight of the catalyst). As well as the catalyst contains carbon nanofiber in form of cylinders of about 3 mm in length and at least 20 μm in diameter in an amount of 1-25% by weight based on the total weight of the catalyst. The method of making the catalyst comprises preparation of the paste which consists of oxide component, carbon nanofiber, boehmite as binder, water, a plasticizer, and a pore-forming component; followed by extrusion, drying, calcinating, and then the consecutive stages of the impregnation with a solution of metal salts are carried out until the content 5-40 wt % of cobalt and 0.1-5% of promoters have been achieved. After each stage of the impregnation the drying and calcinating are performed.
Before carrying out the synthesis, a sample of the catalyst is activated by reduction in the stream of hydrogen (gas hour space rate 100-5000 l/h) at a temperature in the range from 300 to 600° C. during a time period from 0.5 to 5 hours. Synthesis of hydrocarbons from CO:H2 is carried out in a tubular reactor with a fixed bed of the catalyst under a pressure in the range from 0.1 to 4 MPa and a temperature in the range from 150 to 300° C.
However the active component is applied to the support by impregnation, the support comprises the oxide component and metallic aluminium as heat-conducting component. Such procedure results in overconsumption of the expensive active metal to provide the claimed activity and selectivity of the catalyst.
Chin et al. report (Chin Y. H. et.al., Preparation of a novel structured catalyst based on aligned carbon nanotube arrays for a microchannel Fischer-Tropsch synthesis reactor, Catalysis Today, v. 110, pp. 47-52, 2005) a microstructured flat catalyst based on aligned multiwall carbon nanotube arrays for Fischer-Tropsch synthesis in a microchannel reactor. The carbon nanotube arrays are disposed on the surface of metalloceramic alloy foam. Also Chin et al. report the preparation of such catalyst comprising few difficult and time-consuming stages. Preparation method of the structured catalyst with the carbon nanotubes on the surface of the metalloceramic alloy foam involves the following stages:
1) the FeCrAlY intermetallic alloy foam is prepared;
2) the FeCrAlY intermetallic foam is oxidized in air; and then coated with a layer of aluminium oxide by the metal organic chemical vapor deposition (MOCVD) of aluminum isopropoxide at 900° C.;
3) the Fe/mesoporous silica sol is prepared and coated onto the foam;
4) the obtained material is dried and calcinated;
5) the carbon nanotube growth is carried out by catalytic decomposition of ethylene at 700° C.
6) the material with carbon nanotubes is subsequently dip coated with a colloid alumina alcoholic solution, then an aqueous solution containing cobalt nitrate and pherrennic acid to apply Co—Re/Al2O3 onto the material;
7) after each dip drying and calcining at 350° C. for 3 h in air is repeated;
8) the catalyst is activated (or reduced);
9) the catalyst is tested in a microchannel steel reactor.
The carbon nanotubes are fixed on the metalloceramic substrate that provides the efficient heat transfer from the active centers (disposed on the free ends of the nanotubes). However the direction of the heat removal does not correlate anyway with the directions of the reagent and product flows. This fact inevitably negates the efficiency of the heat removal in the reactor. The carbon nanotubes stay out of the mass transfer because the reagents and products are flowed from the free ends of the nanotubes to the outside. It is impossible to use such catalyst in the usual catalytic reactor; this view is substantiated by the author's recommendation to use the catalyst in the microchannel reactor. The stated disadvantages are compensated for the performance features in the microchannel system.
The catalyst has other drawbacks. The catalyst has the sophisticated structural design. The preparation method is quite complicated; it results in quite low technical parameters of the catalyst, namely CO conversion of 42% and CH4 selectivity of 27% . Also that multistage and labor-intensive preparation method makes impossible use of the catalyst in industry.
Therefore the catalysts and the preparation methods described above have the following principal drawbacks: high cost due to high content of the expensive components; low heat conductivity of pellets; complexity of preparation and need of high temperature processing, which require special equipment, increase the catalyst cost and overcomplicates the preparation method.
Numerous studies of the inventors demonstrate that using nanotubes and nanofibers would be efficient if their arrangement in the catalyst gives possibility to enhance the heat flow from the active centers of the catalyst in the direction of the reagent and product flows. Mass transfer intensification would be possible in result of high affinity of the produced hydrocarbons and the graphite-like surface of the carbon nanotubes. The carbon nanotubes have to meet the following criteria to achieve the required effect: firstly almost an ideal cylindrical structure but with enough defects for the intermediate sorption/desorption of hydrocarbons; secondly a length comparable with a path length of a molecule of the product from the active center to the catalyst pellet surface; thirdly, an arrangement along the withdrawal path of the product from the active centers to the catalyst pellet surface, preferably along the walls of the transport pores.
Summarizing the aforesaid it should to be noted that a need exists for an effective, selective catalyst of new type, such catalyst should be stable to overheating, reliable, with improved heat- and mass-transfer in the catalyst pellet and in the catalyst bed as a whole, low-cost along with high activity. As well as there is a need in a simple and safe preparation method of the catalyst.